Cisco IE 4000, Software Configuration Manual

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Configuring STP
Information About Configuring STP
Note: The spanning-tree vlan vlan-id root global configuration command fails if the value necessary to be the root
switch is less than 1.
Note: If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the
switch with the extended system ID support will become the root switch. The extended system ID increases the switch
priority value every time the VLAN number is greater than the priority of the connected switches running older software.
Note: The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an
access switch as the spanning-tree primary root.
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops
between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically
sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can
significantly reduce the convergence time. You can use the
hello keyword to override the automatically calculated hello
time.
Note: After configuring the switch as the root switch, we recommend that you avoid manually configuring the hello time,
forward-delay time, and maximum-age time through the spanning-tree vlan vlan-id hello-time , spanning-tree vlan
vlan-id forward-time , and the spanning-tree vlan vlan-id max-age global configuration commands.
Secondary Root Switch
When you configure a switch as the secondary root, the switch priority is modified from the default value (32768) to
28672. The switch is then likely to become the root switch for the specified VLAN if the primary root switch fails. This is
assuming that the other network switches use the default switch priority of 32768 and therefore are unlikely to become
the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use the same
network diameter and hello-time values that you used when you configured the primary root switch with the
spanning-tree vlan vlan-id root primary global configuration command.
Port Priority
If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the forwarding state. You can
assign higher priority values (lower numerical values) to interfaces that you want selected first and lower priority values
(higher numerical values) that you want selected last. If all interfaces have the same priority value, spanning tree puts the
interface with the lowest interface number in the forwarding state and blocks the other interfaces.
Path Cost
The spanning-tree path cost default value is derived from the media speed of an interface. If a loop occurs, spanning
tree uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces
that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value,
spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.